HP and displacement

[mike76]

Just wondering about how some engines are heavier and have a larger displacement than some smaller ones, yet have the same HP. Whats the deal? Are they both going to be the same HP really? What are you losing with the smaller one?

 

[DoctorCAD]

Re: HP and displacement

Same reason that a 88 cubic inch Harley motor makes 60 HP, but a 90 cubic inch Honda motor makes 85 HP.<br /><br />Design!

 

[DJ]

Re: HP and displacement

Horsepower is now rated at the prop. Has been since 1983.<br /><br />It must meet the hp rating to be called that hp. The spec's. allow a 10% variance, up or down.

 

[phatmanmike]

Re: HP and displacement

some engines have better effiency than others..<br /><br />like the old 1970's merc 150hp inline 6...99 cubic inches.. one of the best hp to ci ratio ever in outboards<br /><br />but the one thing the bigger motor will have hands down every time is T O R Q U E

 

[JB]

Re: HP and displacement

Yup. Like PMM says, the magic word is torque.<br /><br />Peak horsepower ratings are almost as meaningless as advertising slogans.<br /><br />Horsepower is a function of both torque and rpm. Many engineers would prefer engines to be rated according to average torque over a specific range of rpm, rather than peak HP. Here's why.<br /><br />One engine peaks HP at, say, 5,000rpm. A second engine also peaks HP at 5,000, but seriously outperforms the first engine because at 2,3, and 4,000rpm it has more torque (and HP) than the first engine. If the owner of the first engine operates it at WOT all the time he may never know the difference, except when accellerating and the other engine blasts by him.<br /><br />Greater efficiency can cause that. So can greater displacement. An engine that has both will smear another engine with the same rated HP.<br /><br />Peak HP ratings don't tell you that, neither do HP/ci or HP/lb stats.

 

[DJ]

Re: HP and displacement

I call what JB is describing, "seat of the pants".

 

[jim dozier]

Re: HP and displacement

There is an old saying that "there is no replacement for displacement". Given equal engine design and technology, more displacement equals more power. Ah, but there's the rub. The design and technology are often different. Another way of stating what JB said is that it is the area under the power curve (torque or horsepower curve) that is important not the peak. Sometimes you can, with good technology, equal the torque and horsepower of a larger more antiquated engine. This is difficult however, because most of the increase in power in naturally aspirated engines acquired by better design is by increasing the rpm at which torque is available thereby increasing the horsepower (horsepower = torque X rpm X constant). Reciprocating engines are breathing engines and the increased power is obtained by letting the engine succeed in breathing at higher rpms. <br /><br />The exception to this is the addition of forced induction by the use of blowers (turbo/exhaust driven or pulley driven) which allow increase in power at lower rpm such as seen in the new Merc.

 

[james082273]

Re: HP and displacement

This is kind of the same point…..why 2-strokes sometimes feel more powerful than a 4-stroke even though they have the same hp. Some 4-strokes may even be faster on the top end but almost always take longer to get to that speed than there 2-stroke counter part. Though with new technology this is changing especially with forced induction being introduced by Merc. That does wonders for low/midrange torque. <br /><br /><br />A lot of times the smaller displacement engines (or 4-stroke vs 2-stroke) may need to be revved up 250 – 500 rpms higher to reach max hp. What that will do is allow you to use a lower pitched prop which will increase acceleration but still allow you to reach the same speed as the lower RPM higher torque engine. So in other words if you prop correctly displacement is usually a wash. Unless you were to drive one motor right after the other I doubt it would ever make a difference. I personally like to pick the lightest weight for a given horsepower than worry about the displacement.

 

[phatmanmike]

Re: HP and displacement

to elaborate a bit...<br /><br />if you drag race two cars against each other:<br />car 1 weighs 2000lbs, 200hp, 200 CID<br />car 2 weighs 2000lbs, 200hp, 100 CID<br /><br />guess which car will win..... <br /><br />car #1 has more torque , usually aquired through greater displacement.<br /><br />my big inline 6 mercury 1150 is a torque monster, and i wouldnt trade it for the world....<br /><br />and by the way, the new merc VERADOS are inline 6's again. im glad to see merc went back to the roots that made them what they are today( or at least once were)if only my jon boat could hold a 600 pound, 275hp, inline 6, supercharged , 4stroke outboard... id be all over that sh*t

 

[jim dozier]

Re: HP and displacement

The move to smaller lighter higher reving engines in autos is why we have come from the 3-speed manual transmission of the 50's and 60's to the 6-speed manual on some cars today. I look forward to my first 8000 rpm 2-speed shifter outboard.

 

[tee-boy]

Re: HP and displacement

Torque is good!! We like torque around here.

 

[mike76]

Re: HP and displacement

Ok so if you say you'd pick the lightest motor for a given HP, then that Tohatsu 9.8 is looking pretty good then at 81 pounds right? Thats what size engine I am talking about here.

 

[phatmanmike]

Re: HP and displacement

thats for the 4 stroke, the 2 stroke is 52 pounds....

 

[james082273]

Re: HP and displacement

That little 4-stroke 9.8 Tohatsu should do you fine. Just depends on the size of your boat and load. I had a 1444 jon with a 9.9 rude for years. It did everything I ever ask of it. It was always supprising how much of a load that little engine could plane out.

 

[josip_voyager]

Re: HP and displacement

I can't believe that nobody knows nothing about technical issue of engines.<br />There is a formula: ; where P is power in HP (horse power), M is torque in Nm (Newton meter), and w is revolution per minute in rpm. <br />So if you have two outboards with same max. power but with different displacement, outboard with bigger displacement will have better hole shot (acceleration), can carry heavier load, but will have some extra weights. As you can see when power comes from displacement (torque) is almost always better (except for inflatable and smaller boats), because power comes from torque and not from rotation speed. It means that it's much heavier to reach rated power from torque (linear dependence with power) than with rotation speed (square dependence with power). <br />For example Harley Davidson motorcycles have large engines (1200 ccm but only 100 HP at 3.000 rpm), where Japanese have 100 HP motorcycles at 13.000 rpm with 500 ccm displacement.

 

[josip_voyager]

Re: HP and displacement

Excecuse me. Here is formula: P[HP]~ M[Nm]x w^2

 

[JB]

Re: HP and displacement

Thanks for the equation, JV. More simply stated, power = force (torque)X distance/time (RPM).<br /><br />You are not likely to convince anyone that "nobody knows nothing about technical issues in engines" (but you do) by stating that "power comes from torque and not from rotational speed". Your equation contradicts that.<br /><br />Other than that, you have agreed with what was said before.

 

[tee-boy]

Re: HP and displacement

Thanks JB. I will correct you; knowing however that you will probably correct me. I believe Force does not equal torque. However force times distance (revolutions) equals torque; at least that is how my torque wrench measures it. Other than that everything looks good. Simply stated, horsepower is basically how much force can be produced and how many times it can be applied in a given amount of time (a Phd could never explain it that way). Fairly abstract principle but yet so simple and so often confused.

 

[tee-boy]

Re: HP and displacement

Yosip,<br /><br />Your equation is correct but your definitinos don't jive. M can not be NM in your equation b/c you are multiplying it times RPM twice. If you are going to square the RPM term, than M has to represent Newtons and not Newton Meters.

 

[Forktail]

Re: HP and displacement

The correct formula for rotating engines would be HP = (Torque X RPM)/5252.<br /><br />This will give you the HP at any specific RPM, as long as you know the Torque, which is physically measured from a dynomometer. <br /><br />This HP can also be considered WORK, or what the outboard can actually do at that RPM. The total WORK the outboard can do would be represetative of the varying Torque over the entire RPM band. Total WORK is sometimes referred to as the area under the Torque-RPM curve.<br /><br />Two outboards can produce the same maximum HP, but one may be capable of much more work over the entire RPM band.<br /><br />HP (WORK) can be increased either by increasing the RPM, or the Torque.<br /><br />And theoretically, an outboard producing 10 ftlbs of Torque at 5000 RPM will produce the same HP (WORK) as an outboard making 5000 ftlbs of Torque at 10 RPM. Work is Work, even though in some cases it is obtained by using more Torque.<br /><br />Using higher displacement as a rule of thumb for making more Torque and HP doesn't always work. It all comes down to actual engine design, and individual characteristics of how the engine performs WORK. There are many examples of smaller engines making more Torque and HP.<br /><br />It would be very beneficial for outboard manufacturers to provide us with Torque-RPM charts. Personally I am interested in the power (work) at the reduced RPM speeds where I cruise at.<br /><br />Boater Mikey, as far as the 9.8 Tohatsu, you need to remember that that outboard is allowed a +-10% variation in it's 9.8 HP prop rating. This means it could be an 8.8 HP. My guess is that since its displacement is much smaller than other outboards in its class, it doesn't quite get to the 9.8 HP. Only a dynomometer could confirm this.<br /><br />And after all this rotating engine stuff is said and done, we need to remember that it must all be converted to thrust which makes our boats move forward in water. Another whole science dealing with props, gear ratios, water density, water pressure, slip, etc. Prop HP means nothing if it can't be efficiently converted to thrust.